Polyether urethanes containing reactive silane groups, also referred to as silane-terminated polyurethanes (STPs), and their use as sealants and adhesives are known and described, e.g., in U.S. Pat. Nos. 5,554,709; 4,857,623; 5,227,434 and 6,197,912; and WO 02/06367. The silane-terminated polyurethanes may be prepared by various methods. In one method the silane-terminated polyurethanes are prepared by reacting diisocyanates with polyether polyols to form isocyanate-terminated prepolymers, which are then reacted with aminosilanes to form the silane-terminated polyurethanes. The sealants may also be prepared by reacting unsaturated monools with diisocyanates to form intermediates containing unsaturated end groups and then converting these unsaturated groups to alkoxysilane groups by hydrosilylation. In another method the sealants are prepared in one step by the reaction of polyether diols with isocyanatosilanes
To be useful as sealants the silane-terminated polyurethanes should have a number average molecular weight of 6000 to 20,000. One method of obtaining this molecular weight is to use polyether diols prepared by the KOH process and having a molecular weight of 2000 to prepare the isocyanate-terminated prepolymers. The presence of urethane groups causes the products to have a high viscosity. To achieve suitable application viscosities, the high viscosity is reduced by the addition of higher amounts of plasticizer and lesser amounts of fillers, resulting in more expensive sealant products.
Another method of obtaining high molecular weight sealants is by using high molecular weight polyether diols having a low degree of unsaturation and prepared using special catalysts as described in EP-A 0,546,310, EP-A 0,372,561 and DE-A 19,908,562. When these polyether diols are used, the resulting sealants have excellent tensile strength, but the sealants are too brittle for many applications because the elongation is too low and the 100% modulus is too high.
The preparation of sealants from mixtures of polyfunctional and monofunctional silane-terminated polyurethanes is known and disclosed in U.S. Pat. Nos. 5,554,709 and 4,857,623 and WO 02/06367. However, these references do not disclose the use of polyether polyols having a low degree of unsaturation and aspartate-functional silanes to prepare the sealants.
The preparation of silane-terminated polyether urethanes from aspartate-functional silanes is disclosed in U.S. Pat. No. 5,364,955 and WO 98/18843. In both of these references the polyethers used to prepare polyether urethanes do not have a low degree of unsaturation. In addition, mixtures of polyfunctional and monofunctional silane-terminated polyurethanes are not disclosed. Finally, in the latter reference the polyethers must contain 15 to 40% by weight of ethylene oxide units.
WO 00/26271 discloses the preparation of silane-terminated polyether urethanes from polyether polyols having a low degree of unsaturation and aspartate-functional silanes. The products are prepared by reacting diisocyanates with high molecular weight polyether diols to form NCO prepolymers, which are then capped with aspartate-functional silanes to form silane-terminated polyether urethanes. This application does not disclose mixtures of disilane-terminated polyether urethanes with polyether urethanes containing one reactive silane group.
U.S. Pat. No. 6,265,517 describes a similar process for preparing silane-terminated polyether urethanes from polyether polyols having a low degree of unsaturation and aspartate-functional silanes. The patent requires the starting polyol to have a monool content of less than 31 mole %, and teaches that a relatively high monool content is highly undesirable because monools react with isocyanates thereby reducing crosslinking and curing of the prepolymer. The patent also requires the aspartate silanes to be prepared from dialkyl maleates in which the alkyl groups each contain more than four carbon atoms.
EP 0,372,561 discloses polyether urethanes containing reactive silane groups and prepared from polyether polyols having a low degree of unsaturation. In addition, polyether urethanes containing one reactive silane group are disclosed. This application fails to disclose the use of aspartate-functional silanes to incorporate the reactive silane groups.
The deficiencies of the preceding sealants were overcome in copending applications, U.S. Ser. Nos. 10/690,751, 10/690,955, 10/690,956, 10/690,954 and 10/690,931, which describe moisture-curable, alkoxysilane-functional polyether urethanes containing both polyether urethanes having two or more reactive silane groups and polyether urethanes having one reactive silane group. The moisture-curable polyether urethanes are suitable for use as sealants, adhesives and coatings which possess high tensile strengths and elongations and have a reduced 100% modulus when compared with existing products.
In the copending applications the polyether urethane component containing two or more reactive silane groups is prepared from high molecular weight polyether polyols having a low degree of unsaturation. In addition, at least a portion of the reactive silane groups present in at least one of the two components are incorporated by the use of silanes containing secondary amino groups. Finally, the polyether urethane components described in the copending applications are prepared separately and subsequently blended to form the moisture-curable polyether urethanes according to the invention.
Copending application, U.S. Ser. No. 10/690,953 describes a process for simultaneously preparing moisture-curable polyether urethanes containing a mixture of polyether urethane component having two or more reactive silane groups and a polyether urethane component having one reactive silane group instead of being prepared separately and mixed. The mixture of polyether urethanes retains all of the valuable properties of the polyether urethanes disclosed in the previously described copending applications.
A disadvantage of the moisture-curable polyether urethanes described in the preceding copending applications is that they are not storage stable at moderately elevated temperatures of 50 to 90° C., which may be present in a drum stored in a hot climate.
Accordingly, it is an object of the present invention to provide a process for preparing moisture-curable polyether urethanes at lower production viscosities, in which the resulting products are storage stable at elevated temperatures and also retain all of the valuable properties of the polyether urethanes disclosed in the preceding copending applications, i.e., the products are suitable for use as sealants, adhesives and coatings which possess high tensile strengths and elongations and have a reduced 100% modulus.
This object may be achieved with the process of the present invention in which the moisture-curable polyether urethanes containing a mixture of polyether urethane component having two or more reactive silane groups and a polyether urethane component having one reactive silane group are prepared simultaneously and in which the polyether urethanes contain terminal cyclic urea/reactive silane groups.
The present invention is based on the surprising discovery that the non-cyclic urea groups formed by the reaction of isocyanate groups and aspartate silane groups decompose back into the starting components when stored at moderately elevated temperatures. In accordance with the present invention this decomposition is prevented by converting the non-cyclic urea groups into cyclic urea groups, which are thermally stable.
It is surprising that the polyether urethanes obtained according to the process of present invention can be used to prepare cured resins that possess the same properties as those obtained in accordance with the copending applications because the presence of cyclic urea groups would be expected to result in less flexible cured resins that would not have the same elongation and 100% modulus as cured resins prepared from polyether urethanes containing non-cyclic urea groups.